This invention relates to light sources and in particular though not exclusively to optical transmitters.
Optical transmitters have an important application in optical communications systems employing optical fibres as transmission medium.
One of the major obstacles to the practical implementation of optical communication systems is the difficulty of achieving sufficiently accurate alignment in and of the various system and subsystem components, and maintaining that alignment for extended periods which may be as long as the lifetime of the components.
The difficulty stems from the small dimensions of the components concerned. Particularly where transmission over single mode fibre is concerned, even minute misalignments of the order of a micrometre or less can cause noticeably increased transmission losses.
The active region of a typical semiconductor laser is of the order of 2 .mu.m in cross-section. The core of a single mode fibre is about 5 .mu.m to 10 .mu.m in diameter. In a conventional laser transmitter, the active region needs to be aligned with the fibre core. For practical reasons it is often necessary also to space the end of the fibre by a small distance, typically 20 to 25 .mu.m for a lensed fibre end, from the adjacent laser face. This reduces the required accuracy of alignment in practice to about 1 to 2 .mu.m if excessive coupling losses between the laser and the fibre are to be avoided.
A further alignment is usually required between the back facet of the laser, that is to say the laser facet opposite that facing the fibre, and a monitoring photodetector. The usually much larger active area of the photodetector, typically 250 .mu.m or more in diameter for a slow speed monitoring device, relaxes the required accuracy of alignment somewhat. However, light needs to be incident on the active area from above (or below) the major plane of the device. This is achieved conventionally by mounting the photodetector with its major plane perpendicular to the active region of the laser. This approach is adopted despite the practical difficulties of handling a detector device in that orientation during assembly, and finding a satisfactory way of making electrical connections to the photodetector, for example. Problems of this nature adversely affect production costs, and present an obstacle to high volume production in particular.